Just block them, change your number if you must, send unopened mail back via “return to sender”. If you must send a last written request, get your attorney to. Don’t try handling this relationally. Consider it a legal issue. They aren’t going to change. Now, just so you know, 2 of my adult children went no contact with me strangely both are close to your age. I respect their decision but most won’t. I don’t agree I was abusive but I don’t question the validity of their perspective. I love my children and know I am me and they decided I was bad for them. Better just do what is best for you and don’t expect them to understand.